1. Field of the Invention
The present invention relates to a method for producing a photovoltaic device comprising a photovoltaic cell including at least one film of a semiconductive metal oxide.
2. Description of the Related Art
Semiconductive metal oxides such as oxides of titanium, zirconium, hafnium, strontium, zinc, indium, yttrium, lanthanum, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, show a crystalline phase exhibiting photocatalytic functionality, and can be use in a variety of applications. For example, Titania (TiO2) has several crystalline phases such as anatase, rutile and brookite. Among these crystalline phases, the anatase one exhibits higher photocatalytic properties and photovoltaic effect and thus has attracted much attention in these fields.
Nanosized anatase titania film have been studied for applications such as solar cells, photocatalysts, antibacterial coating, electrochromic display, anti-reflecting coating and gas sensors. The photovoltaic devices fall within the general class referred to as dye-sensitized solar cells, as reported, e.g., by U.S. 2003/0188776.
Presently, many applications would benefit from the availability of a photovoltaic cell or module, for example, the so-called “smart cards”, i.e., electronic cards capable of storing information and used, e.g., for payphone telephony, digital mobile telephony, the credit and debit functions of financial institutions, retail loyalty schemes, corporate staff systems, subscription TV operations, mass transit ticketing schemes and the like.
Typically, these cards comprises an information module, which provides information (e.g., electronic data or an audiovisual response) to a user or to a card reader, and a photovoltaic cell that powers the information module. The information module and the photovoltaic cell are typically supported on a polymeric substrate.
Thin films based on nanoparticles of anatase titania show high photocatalytic activity depending on phase, crystal dimension and surface area, and porosity. As reported by Yoshinori Kotani et al. Journal of Sol-Gel Science and Technology 19, 585-588, 2000, the sol-gel method is one of the most promising techniques to prepare thin films because it has a number of advantages such as low-temperature processing and the ability to prepare materials in various shapes, compared with the conventional preparation procedures of glass and ceramics. However, as-prepared films by the sol-gel method are usually amorphous, and a high temperature process over 300° C. is required to form anatase nanocrystals. Therefore, it is difficult to form anatase nanocrystals on the substrates with poor heat resistance such as organic polymers.
EP-A-0 859 385 (in the name of Monsanto Company) discloses a method for manufacturing photovoltaic cells comprising polycrystalline oxides exhibiting semiconductor functionality. Particles of the polycrystalline metal oxide can be prepared by hydrolysis of the corresponding metal alkoxide followed by optional physical treatments such as growth and particle size control through digestion under hydrothermal conditions at temperatures in the range of from 150 to 250° C., followed by high temperature (200-500° C.) sintering and grinding of the resulting sintered product to the required particle size. Said particles are then dye coated and suspended to yield a ink suspension, optionally containing additives, e.g. dispersants which can enhance the even distribution of the ink particles on the substrate where the ink suspension is deposited to yield a uniform layer. The so deposited layer is treated under mild and non-destructive conditions including temperatures below 180-150° C., possibly combined with non-destructive pressure e.g. below 20 bars, and/or evaporation under sub-atmospheric pressures. No specific examples are provided.
EP-A-1 167 296 (in the name of Kawasaki Jukogyo Kabushiki Kaisha) relates to a process for producing anatase titanium oxide having photocatalytic activity and large specific surface area. Anatase particles are prepared by a sol-gel method starting, for example, from a metal oxide or alkoxide heat treated in a closed vessel in the temperature range of 80 to 250° C. The examples show that a temperature of about 240° C. is necessary to obtain the anatase phase while operating at atmospheric pressure.
EP-A-1 182 169 (in the name of Japan Science and Technology Corporation) relates to a process for producing anatase titania or composite oxide containing anatase titania wherein a gel containing a metal oxide is formed from a solution containing a hydrolysable titanium compound and an organic polymer (e.g. polyethylene glycol), and subsequently the gel is allowed to react with water at a temperature of 100° C. or below.
Matsuda A. et al., J. Am. Ceram. Soc., 83 [1], 229-31, 2000 describe the preparation of transparent anatase nanocomposite films on various type of substrates, including organic polymers, using a sol-gel method at temperatures lower than 100° C. under ambient pressure. The homogeneous dispersion of titania particles in the matrix and control of the porosity cannot easily be attained. In the preparation of titania porous films, the particles have a tendency to aggregate in the sol and the resultant films usually become opaque. A large surface area, high transmittance of ultraviolet (UV) light and durability for photocatalytic activity are required for the host matrix in which anatase nanoparticles are dispersed without aggregation. This document proposes silica gel as matrix candidate. It is remarked that the formation of anatase nanocrystals is hardly observed in pure titania and is a unique phenomenon to the silica-titania system.